Very often, in circuit applications, it is necessary to provide high values of current, in the order of hundreds of amperes, to a load. The current demand of a welding machine is a good example. In recent years, it has been the practice, very often, to employ a power silicon controlled rectifier (SCR) as a switch to control such high current demands. The current conduction of an SCR must be terminated by effectively terminating current conduction at the anode or cathode thereof. Such termination is very often accomplished by employing a control SCR which has an accompanying trigger circuit and a power SCR with an accompanying trigger circuit. The trigger circuits are fired after a predetermined R-C time delay and the circuit has a capacitance coupling between the power SCR and the control SCR so that when one SCR is turned on, it turns off the other SCR.
While this technique has been satisfactory to some extent, it has failed to effect termination at times if current transients are created. These current transients can provide a sufficient amount of current to keep an SCR conducting through the termination time period and if the SCR is conducting thereafter, then both SCRS are conducting and there is no means to turn either SCR off. The present circuit is arranged to terminate the operation of each power SCR by having a current conduction control SCR circuit repeatedly render the anode of the power SCR sufficiently negative to effect termination of current conduction. If a power SCR should be maintained in a conducting state by a transient, the current conduction control SCR will get repeated subsequent chances to turn it off and will effect said termination during one of those efforts.